In recent years, a storage apparatus which configures a great number of logical volumes in a Redundant Array of Independent Disks (RAID) has spread by increase of the storage capacity of physical disks (storage devices) which configure the RAID.
As such a storage apparatus, an apparatus is available in which I/O (Input/Output) requests such as writing requests for a plurality of logical volumes in the same RAID are processed in parallel. As a reason why a parallel process of I/O requests can be performed, it is listed that the performance of hardware of an interface between a host apparatus and a storage apparatus is enhanced or that the redundancy is provided by a plurality of paths.
It is to be noted that, in a storage apparatus, data relating to an access request such as an I/O request is stored into a cache memory and a write back process is performed in which dirty data before execution of write back is discharged (written out) from the cache memory to a physical disk by the write back method, for example. The write back process can be performed, for example, periodically or in the case where capacity shortage (cache depletion) of a free cache memory occurs and therefore a cache region is released.
[Patent Document 1] Japanese Laid-Open Patent Application No. 2005-258789
[Patent Document 2] Japanese Laid-Open Patent Application No. 05-303528
In a storage apparatus, a plurality of logical volumes which are an access target of a write request issued from a host apparatus are sometimes allocated to one physical disk. In this case, if the free capacity of the cache memory becomes short, then write back of dirty data is performed. However, if a plurality of the write back processes are executed by a parallel process of write requests, then the processing performance of the physical disk sometimes becomes insufficient with respect to the data amount relating to the write back.
Consequently, time is taken to secure the free capacity of the cache memory and a delay may occur in allocation of the cache memory with respect to the write request in the writing process.
It is to be noted that, not only in the writing process but also in an access request, for example, in a readout request, a delay may possibly occur in allocation of a cache memory with respect to an I/O request similarly as in the writing process. This is because, in a readout process relating to a readout request, if the free capacity of a cache memory into which data of a readout target to be readout from a logical volume is to be stored becomes insufficient, then write back occurs.